This invention relates to an electrode incorporating a novel sensor film and used for the measurement of the activity of an ion in a solution, and to a method for the manufacture of said sensor film.
Generally the activity of an ion in a solution is measured by the following method.
A reference electrode and an electrode designed for the measurement of ionic activity are immersed and opposed to each other in a solution under test and the potential difference between these two electrodes is measured, whereby the activity of the ion in the solution is determined on the basis of the measured value of said potential difference.
Now, the construction of a typical liquid film type electrode designed for the measurement of ionic activity will be described below.
A porous thin film formed of a chemically stable substance is attached over the opening at the bottom of a tubular electrode body. Inside the body is placed an internal solution and an organic ion-exchange liquid which acts as a sensitive substance. This latter mentioned liquid is a substance which is immiscible with the solution under test and which when held in direct contact with said solution, induces the phenomenon of ion exchange along the interface. This liquid permeates the porous thin film so that when the electrode is immersed in a given solution for the purpose of measurement, the solution under test and the internal solution of the electrode come into mutual contact through the medium of the organic ion-exchange liquid.
The electrode disclosed in Japanese Patent Publication No. 14678/1969, (U.S. Ser. No. 390016, filed Aug. 17, 1964, now U.S. Pat. No. 3,429,785) the Model 92 electrode made by Orion Corp. of the United States, the Model 476114 electrode made by Corning Corp. of the United States, etc. are all electrodes of the construction described above, designed for the measurement of ionic activity.
These electrodes have disadvantages such as are indicated below.
1. It is impossible to completely prevent the organic ion-exchange liquid from passing through the porous thin film and entering into the solution under test.
For this reason, the organic ion-exchange liquid must be kept under constant watch during the service of the electrode and must be replenished to make up for the loss due to the escape into the solution.
2. The escape of the organic ion-exchange liquid into the solution under test entails a degradation in the accuracy of measurement.
3. Where the solution subjected to measurement contains an organic solvent such as alcohol, the organic ion-exchange liquid and the solution are dissolved into each other, making the measurement impossible.
4. The electrode construction is complicated. The electrode made by Orion Corp., for example, is composed of a portion filled with the organic ion-exchange liquid, a portion filled with the internal solution of the electrode, a portion serving to retain in position the porous thin film, an internal electrode, etc. Besides, the electrode is required to be constructed so as to permit replacement of the organic ion-exchange liquid or porous thin film. For these reasons, the electrode is manufactured at a great cost.
5. The porous thin film has insufficient mechanical strength. If the solution under test happens to contain any floating matter, there is a possibility that the thin film will sustain injuries. The solution under test is normally kept in a stirred condition during the measurement in order to ensure high accuracy of the measurement. The resulting agitation of the solution, however, causes vibration of the thin film and accordingly prevents the electrode from registering results with high consistency.
6. Because of deterioration, the thin film must be replaced with a new one at certain intervals. The detachment of the old film and the attachment of the new film require high skill on the part of the operator.
All the defects enumerated above originate from the fact that a porous thin film is used as the sensitive film.
To overcome these drawbacks, there have been proposed and accepted for actual use improved sensitive films such as, for example, a film supported on a porous carrier made of colloidal silica and a film produced by mixing a liquid sensitive substance with a synthetic polymer like an epoxy resin or silicone rubber, molding the resultant mixture and causing the molded mixture to set.
In the former film, although the mechanical strength is improved to some extent, the other defects still persist.
The latter film is difficult to manufacture and possesses insufficient mechanical strength and, because the film surface is not smooth and the film texture is not hard, the film provides a short service life and fails to maintain its precision of measurement stably for a long time.
An object of the present invention is to provide an electrode for the measurement of ionic activity, which enjoys high mechanical strength, provides a long service life, permits measurement of various kinds of ions and excels in precision of measurement.
Another object of the present invention is to provide a method for the manufacture of a sensor film for the electrode to be used for the measurement of ionic activity, which electrode enjoys high mechanical strength, provides a long service life, permits measurement of various kinds of ions and excels in precision of measurement.